A wire electric discharge machine is arranged to generate electric discharge between a wire electrode and a workpiece while the wire electrode runs under a tension, to perform machining. During electric discharge machining, the wire electrode may be undesirably broken in a workpiece. In this case, the wire electrode must be rewound and connected. Conventionally, wire electrode rewinding is performed by rotatively driving a feed reel while the feed reel is being torque-controlled by an AC motor or the like. However, even when the AC motor is driven with its output torque-controlled, its rotational speed cannot be controlled. Therefore, the feed reel rotates at a high speed and it is idling after the wire electrode rewinding is completed. As a result, a winding state of the wire electrode of the feed reel is degraded. This is inconvenient to connect the wire electrode. Disclosure of Invention
It is an object of the present invention to provide a wire feed mechanism of a wire electric discharge machine, which can rewind a wire electrode, that is broken during electric discharge machining, on a feed reel, without disturbing its winding state, and which can provide a rewinding state convenient for subsequent connection of the wire electrode.
In order to achieve the above object, the present invention comprises a clutch having clutch elements one of which is integrally coupled to the wire electrode feed reel and rotatable therewith and arranged to create a coupling torque between the clutch elements that changes in accordance with a feed voltage, and a geared motor coupled to the other clutch element of the clutch. Preferably, the present invention further comprises detecting means for detecting a breakage of the wire electrode and outputting a breakage detection signal, and control means responsive to the breakage detection signal for supplying a first predetermined voltage to firmly engage the clutch, and for supplying a second predetermined voltage to loosely engage the clutch and driving the geared motor after rotation of the feed reel is stopped.
In this manner, according to the present invention, a wire electrode feed reel is driven by a geared motor through a clutch creating a variable coupling torque between clutch elements, i.e., input and output members thereof. Therefore, during electric discharge machining, a feed voltage applied to the clutch is controlled to control the tension of the wire electrode to a predetermined value. Further, when the wire is broken, the rotating feed reel and the stopped motor are coupled through the firmly engaged clutch. As a result, rotation caused by the inertia of the feed reel and so on can be immediately stopped, and the feed reel may not idle to loosen the wire electrode. Furthermore, the wire electrode is rewound by the geared motor through the loosely engaged clutch at a predetermined torque and at a low speed. Therefore, a breakage point can be easily detected, and the electrode can be stopped at a position convenient for subsequently connecting the wire electrode.